Your burgers look and sound delicious. Anything I would have suggested has already been mentioned in your comments, so I hope you'll find something that works. I'll be having a Dr. Praeger California Veggie Burger for lunch today, but those stay together well... most likely 'cause of the preservatives.

NO! I'm wrong... I just got curious and checked the ingredient panel on the Praeger burgers! I'm impressed!
Ingredients
Carrots, Onions, String Beans, Oat Bran, Soybeans, Zucchini, Expeller Pressed Canola Oil, Peas, Broccoli, Corn, Soy Flour, Spinach, Red Peppers, Arrowroot Powder, Corn Starch, Garlic, Corn Meal, Sea Salt, Spices